In Long Term Evolution (LTE) it is possible for an eNodeB to dynamically schedule User Equipments (UEs) in downlink and uplink by transmitting downlink scheduling assignments and uplink scheduling grants on the Physical Downlink Control CHannel (PDCCH) on 1 ms basis.
For uplink transmission a UE that has data to transmit but has not been given any uplink grant can indicate to its serving eNodeB that it would like to transmit data by sending a scheduling request on the Physical Uplink Control CHannel (PUCCH).
For downlink transmission, scheduled data is transmitted on the Physical Downlink Shared CHannel (PDSCH) in the same subframe, which is 1 ms long in time, in which the downlink assignment is transmitted on PDCCH.
The UE can then transmit a Hybrid Automatic Repeat reQuest (HARQ) ACKnowledgement (ACK) or Negative ACKnowledgement (NACK) response on PUCCH, based on whether the UE received data successfully on PDSCH.
For dynamic scheduling, the resource on PUCCH to be used for the ACK/NACK response is given by the PDCCH resource on which the downlink assignment is transmitted.
There is a requirement of a low load for PUCCH, based on studies that have shown that the usage of the available PUCCH resources should be kept around 20%. In the uplink, the UE transmits on the Physical Uplink Shared CHannel (PUSCH) a certain number of subframes after having received an uplink grant. The eNodeB transmits the HARQ ACK/NACK response on the Physical HARQ Indicator CHannel (PHICH). The resource for the ACK/NACK response is given by the PUSCH resource allocation and the demodulation reference signal cyclic shift.
In the uplink the HARQ is synchronous, which means that it is known to the UE in which subframe to transmit a retransmission. If the UE does not receive a grant from the eNodeB for a retransmission, the retransmission will be transmitted on the same resources in frequency as for the previous transmission attempt. If a grant is received, the retransmission is done on the frequency resources which are indicated in the new grant. It is possible for the eNodeB to cancel a synchronous HARQ retransmission by transmitting an ACK on PHICH. The available PDCCH resources for grant or assignment signaling are limited, and so are the PUCCH resources for scheduling request signaling.
In order to limit the grant or assignment signaling needed, as well as the scheduling request, a concept called Semi-Persistent Scheduling (SPS) has been introduced in LTE. In semi-persistent scheduling an assignment or grant is valid with a certain periodicity which is configured by higher-layer signaling, Radio Resource Control (RRC).
For downlink SPS are the PUCCH resources that the UE can use for HARQ ACK/NACK responses also RRC configured. The semi-persistent scheduling can be initiated by sending a grant or assignment with a special Cell Radio Network Temporary Identifier (C-RNTI). In downlink, the initial assignment contains information about which one of the 4 RRC configured HARQ ACK/NACK resources that should be used. SPS can be deactivated by sending a grant/assignment with special C-RNTI.
It is up to the eNodeB scheduler to decide when to transmit the initial grant or assignment. The straight forward solution for when to do this is to do it as soon as 1) there is an indication of that there is data to transmit for the service for which SPS has been configured and 2) there are available resources on PDCCH to transmit the assignment/grant and 3) available resources on PDSCH or on PUSCH to transmit the data.
Within the existing solution for when to transmit the initial grant/assignment the SPS transmissions might become unevenly distributed over time. In subframes with a lot of SPS transmissions there might be few or no PDSCH resources in case of downlink SPS or few or no PUSCH resources in case of uplink SPS for dynamically scheduled data. Having subframes with high SPS load also means that there are subframes with a lot of HARQ ACK/NACK responses to the SPS transmissions on PUCCH in case of downlink SPS or on PHICH in case of uplink SPS. Moreover, having subframes with high SPS load also means that there are subframes with a lot of HARQ ACK/NACK responses to the SPS transmissions on PHICH in case of uplink SPS.
There are thus subframes having a high load on PUCCH unless the number of users which can be scheduled in downlink dynamically in the same subframe is limited, considering the 20% load requirement on PUCCH.
Similarly, there can be subframes having a high load on PHICH unless the number of users which can be scheduled in uplink dynamically in the same subframe is limited.
One alternative way to solve the PUCCH high load problem is to over-dimension the PUCCH. The PHICH load problem may be solved by dimensioning the PHICH such that the number of PUSCH resources is always more limiting than the number of PHICH resources. However, as a consequence to over-dimensioning the PUCCH resources, the overhead is increased.
Potential problems with prior art techniques are:                Scheduling of other data has to be postponed which is bad for data with high delay requirements,        High PUCCH load or high PUCCH overhead,        High PHICH load,        Waste of PDCCH resources, since there are unused PDCCH resources in the subframes with SPS transmissions occupying a lot of the PDSCH or PUSCH resources,        Interference variations in time, and        Synchronous HARQ retransmission and SPS transmission collisions in uplink which have to be resolved by sending grants to deactivate SPS or move SPS transmissions in frequency, or grants to move the HARQ retransmissions in frequency or ACKs on PHICH to cancel the HARQ retransmission.        
There is thus a need for an improved method and arrangement of load management for eliviating at least some of the potential problems above.